Ensure Qt Quick Controls 2 is fully compatible with the QRhi-based rendering path

Is there something needed still in quickcontrols2? Apart from Material style patch which is merged already in 5.14. There is some shader code in an autotest at least, do we care about that?

Update: Imagine style relies on graphicaleffects in about two places. These effects are currently missing when rendering via QRhi is enabled.
-> https://codereview.qt-project.org/c/qt/qtquickcontrols2/+/299971 adds an own OpacityMask effect implementation so no Graphical Effects dependency is needed anymore.

In the autotests there are two places where Graphical Effects is used in a scene in the test data. None of these are fatal, apparently: failling to apply the effect (due to the lack of an RHI-compatible shader) still makes said tests pass. The dependency should be removed nonetheless, given that Graphical Effects may not ship with Qt 6.0.

Some autotest cases use ShaderEffect (e.g. in the Material style's tests). Strangely enough these, while fail to initialize, also still lead to a passing test, so these won't be immediate blockers either.